Nonlinear loads such as switching power supplies used in personal computers, variable frequency drives (VFDs), and other electronic devices can generate excessive harmonic currents in five-phase distribution systems, including zero phase sequence harmonics, in commercial and industrial power distribution systems.
In a five-phase power distribution system, fifth harmonics and multiples of fifth harmonic are of zero-sequence nature. Having zero sequence harmonics often results in high voltage and current distortion, neutral conductor overloading, motor and power transformer overheating, increased losses, and excessive harmonic injection onto the utility supply system (e.g., power grid). Zero-sequence current flowing in the neutral conductor of the five-phase six-wire distribution power system is five times of the zero sequence harmonic components of each phase current (e.g., the sum of the zero sequence harmonic components on each phase). Therefore, zero-sequence current has the potential to cause main power quality problems. There is a need to mitigate the zero-sequence harmonic current.
There are several existing arrangements used to mitigate zero sequence currents in five-phase distribution system. One arrangement uses passive LC (inductor-capacitor) filters generally with capacitor-inductor combinations, as is widely used in the power systems harmonics mitigation. The passive filter LC filter removes some neutral harmonic current and is comprised of series and/or parallel capacitors and resonant inductors. These tuned circuits provide low or high harmonic impedance pathway for harmonics in five-phase distribution system. The filtering characteristic of the passive LC is determined by the ratio of filter impedance and system impedance. Other than the filtering characteristics, the passive LC filters have the following drawbacks:                LC filters are prone to be influenced by system parameters;        While LC filters remove/block several specific harmonics, LC filters amplify some of the harmonics; parallel series resonances        If the harmonic current increases, there is a potential for the LC filter to overload;        Capacitor values change with the aging, resulting in the LC filters becoming less effective at the engineered harmonic frequencies.        
With the development of power electronics technology, active filters have been implemented in place of LC filters. Active filters use power semiconductor devices that inject current to the network, with equal in amplitude but 180 degrees out of phase with the system harmonic current. When the out of-phase signal is added with the system harmonic current, the total harmonic current approaches zero. Although the active filter's performance is better than passive filter, its circuit topology, control complexity, high cost, and low reliability limits usefulness.
What is needed is a five-phase filter system that will reduce fifth harmonics with reduced costs and improved reliability.